Since the invention of the first laser many years ago, the frequency conversion off laser radiation by nonlinear optical crystals has become an important technique widely used in quantum electronics and laser physics for solving various scientific and engineering problems. The fundamental physics of three-wave light interactions in nonlinear optical crystals is now reasonably well understood. This has enabled the production of various harmonic generators, sum- and difference-frequency generators, and optical parametric oscillators based on the nonlinear optical crystals that are now commercially available. At the same time, scientists continue an active search for improved nonlinear optical materials. The present invention relates to a method for the fabrication of a bulk periodically poled domain (PPD) structure-in congruently grown lithium niobate (LN), Lithium Tantalate (LT) and Magnesium Oxide doped LN and LT (MgO:LN and MgO:LN). These structures comprise nonlinear optical materials which are suitable for use in optical devices to convert near infrared (NIR) radiation from a diode or other laser to light in the blue-green (visible) or near UV region of the optical spectrum. In particular this invention relates to an efficient process for the fabrication of poled Lithum Niobate and Lithum Tantalate crystals for the generation of blue-green and UV light using quasi phase matching (QPM). These materials are ferroelectric, which means that below their Curie temperature they exhibit a spontaneous electric polarization even if no external electric field is applied. Periodic inversion of the spontaneous polarization is called “poling” and provides a means for producing the 180° phase shift required to implement QPM. Further information on the applications and properties of nonlinear crystals, especially LN and MgO doped LN, are given in “Handbook of Nonlinear Optical Crystals” by V. G Dmitriev, G. G Gurzadyan and D. N. Nikogosyan Springer-Verlag (1999) ISBN3-540-69354-5292.
Laser light in the blue-green (visible) wavelength (i.e., 400-550 nm) wavelength region is used in a wide variety of analytical techniques. Currently the only generally available source of coherent blue-green light is the Argon ion laser. Argon ion lasers are relatively bulky, delicate and expensive. There is a great need for a solid state blue-green laser. None is currently available which emits in this spectral region. However there are solid state (semiconductor diode) lasers which emit in the near infra-red (NIR) region.
Frequency doubling (nonlinear frequency conversion) would enable a NIR laser to provide blue-green light. An extensive discussion of blue-green laser technology and applications, including biomedical engineering, spectroscopy, semiconductor wafer inspection, display science, optical data storage, reprographic, color display and undersea communication is contained in “Blue-Green Lasers” by W. Risk, T. Gosnell and A. Nurmikko, Cambridge University Press (2003) ISBN 0-521-52103-3. The discussion of nonlinear frequency conversion using quasi phase matching in nonlinear crystals contained therein is incorporated herein by this reference (see especially pages 77-84 and 101-104 and 108).
Conversion of NIR laser source light into light in the green-blue spectral range can be carried out by using second harmonic generation (SHG), also known as frequency doubling techniques, a technology generally known in the laser-based optical industry. A reasonable goal for single-pass conversion efficiency is that it should be in ˜25% range to avoid excessive laser cost. Conversion efficiency is proportional to input power, the square of the effective nonlinear coefficient of the nonlinear element (crystal) and the length of the nonlinear element As crystal length is increased, conversion efficiency increases, but the frequency doubling process becomes more sensitive to changes in temperature, strain and other factors affecting the uniformity of the refractive index of the non-linear element. As a result, length alone cannot be used to compensate for an element's low nonlinearity.
A technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion is known as birefringent phase matching. In this case, the optical anisotropy of a nonlinear crystal is used to find a unique propagation direction where fundamental and harmonic waves have the same phase velocity. For most of the commercially available nonlinear optical materials (LN, LT, and KTP) the maximum conversion efficiency is about 1.25%. LN and LT are particularly attractive materials because of their status as commodity materials, and as follows from Table 1 they have a high nonlinear coefficient (normally referred to as d33) and also are available in relatively large size crystals.
Quasi-phase matching (QPM) provides the mechanism for an efficient way to generate second harmonic frequencies. The general concept of using QPM as a mechanism for doubling optical frequencies has been known for about forty years. Essentially it is a technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion. In QPM, the two waves are allowed to have different phase velocities, and they shift out of phase relative to one another over a distance called the coherence length
At present the most efficient way to create a QPM structure is to use periodically poled crystalline ferroelectric materials. In these materials, creating specific micrometer scale domain configurations with a periodically alternating direction of spontaneous polarization, are used for this purpose. Due to the polar character of these materials the sign of the non-linear coefficient (d33) can be changed by switching the direction of spontaneous polarization. If the period (Λ) of the periodically polled domain structure is equal to double the coherent length, the phase difference due to natural dispersion is compensated for by the change of the sign of the non-linear coefficient (d33−d33) at the domain boundaries, causing the continuous transference of power from the fundamental beam to the harmonic beam throughout the entire length of the crystal.
The efficiency of a conversion is a strong function of both the non-linear coefficient and the length of the crystal Therefore, to enhance the efficiency of conversion the material with the maximum non-linear coefficient should be used, and the interaction region should also be as long as possible.
Effective nonlinear coefficient value, poling period and absorption edge (the range of limited absorption) are the factors which influence the choice of periodically poled materials for SHG applications.
There are only a few ferroelectric materials with nonlinear coefficients greater than 10 pm/V (as shown in Tab. 1). While dispersion prevents direct access to the full value of d33, QPM can provide up to (2/π)×100%=64% of the full nonlinearity, making each of the materials shown in Table 1 a prima facie candidate for efficient doubling of the frequency of semiconductor lasers.
TABLE 1Parameters of perspective ferroelectrics for QPM-SHG applicationsCurieCoercived33TemperatureField, EcGrowth method andCrystalpm/V° C.KV/mmmax. crystal sizeLiNbO340 ± 5~1160≦2*CZ(LN)20-25**100Φ × 60 mmLiTaO320 ± 2610≦2*CZ(LT)18-25**100Φ × 50 mmKNbO325 ± 5225≦0.6CZ(KN)15Φ × 20 mmKTiOPO415 ± 2~6702.6CZ(KTP)20Φ × 25 mm*For stoichiometric LN and LT**For congruent LN and LT
Table 1 shows that most effective material for SHG application i.e., the material which has the biggest value of d33 is a LN crystal. Due to QPM, it is possible to create viable bulk single-pass blue-green light sources using LN, since it provides a way to obtain a normalized room temperature conversion efficiency of 2.6-3% /(watt•cm) for 1064 nm532 nm SHG and 5.2%/(watt•cm) efficiency for 852 nm426 nm SHG. LT has a normalized room temperature conversion efficiency of 0.85-1%/(watt•cm) for bulk single-pass 1064 nm532 nm SHG. For 852 nm426 nm SHG, LT has a normalized conversion efficiency of 2%/(watt•cm). It should be noted that for wavelengths of ≦410 nm LT has higher transparency compared to LN and therefore it may be the material of choice for 300 nm-410 nm wavelength (near UV) conversion . Two materials in which QPM has been demonstrated for blue green (BG) light generation are KN and KTP. The normalized conversion efficiency for these materials is ˜2.5%/(watt•cm) for KN and 1.5%/(watt•cm) for the 852 nm426 nm SHG. To achieve 25% single-pass conversion efficiency, a KN crystalline element of 2.5 cm and a KTP crystalline element of 3.5 cm length is required. However the maximum crystal length currently available is 2 cm for KN and 3 cm for KTP. Therefore, KN and KTP are currently not viable choices because both crystals are not commercially available in sufficiently long sizes, are very expensive, and also suffer from a number of quality issues. Lithium niobate (LiNbO3), often referred to as “the silicon of nonlinear optics,” is an excellent material for SHG for two reasons. First, LiNbO3 is already produced at a volume of 40 tons per year for consumer applications (cellular phones and televisions) using a very stable fabrication technology. Second, LiNbO3 is transparent from 350 nm to 5000 nm, providing low loss for both the fundamental and harmonic for visible light generation. Finally, LiNbO3 has nonlinear coefficients for visible light generation among the highest of all inorganic materials.
While LiNbO3 is an attractive material because of its status as a commodity material, the only component of its nonlinear tensor large enough to satisfy the requirements of display applications is d33, having a value of 25.2 pm/volt. While dispersion prevents direct access to the full d33 coefficient, quasi-phase-matching (QPM) can provide up to 64% of the full nonlinearity, or 16 pm/volt, making LiNbO3 a very strong candidate for display applications that use QPM.
Essentially QPM is a technique that compensates for the difference in phase velocity between the fundamental wave and its harmonic in a nonlinear crystal caused by natural dispersion. In QPM, two waves having different phase velocities shift [pi] out of phase relative to one another over a distance called the coherence length. The sign of the nonlinear coefficient reverses every coherence length, causing the locally generated harmonic field to transfer power to the harmonic beam. By compensating for phase-velocity mismatch in this way, all elements of a crystal's nonlinear tensor can be accessed throughout the entire transparency range
Three other potential materials in which QPM has been demonstrated for visible light generation are LiTaO3 and MgO doped LN and LT. LiTaO3 has a normalized room temperature conversion efficiency of 0.83%/(watt-cm), below that required for bulk single-pass 1064 nm SHG. However, for 852 nm SHG, LiTaO3 has a normalized conversion efficiency of 1.8%/(watt-cm) and is therefore a strong candidate for that application.